Stabilization of cellulose nitrate



Patented Apr. 20, 1937 UNITED STATES STABILIZATION OF Louis S. Baker, Parlin,

CELLULOSE NITRATE N. J., assignor to E. I. du

Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing.

4 Claims.

This invention relates to a process for reducing the quantity of residual acid in cellulose nitrate and more particularly to a process for the purification and stabilization of cellulose nitrate by reducing the residual acidity and the product produced by such a process.

Cellulose nitrate as produced by present commercial practice contains unstabilized acid bodies which decompose on storage. The decomposition products of these unstable bodies apparently react with the cellulose nitrate itself to form more unstable bodies by further decomposition on storage and the instability of the cellulose nitrate thus becomes progressive. It is, therefore, necessary that a process of stabilization be applied to each lot of cellulose nitrate and that a careful control of such stability be made since, as is well known, some types of cellulose nitrate products are required to be stored for long pe- 0 riods of time before being used. An example is smokeless powder.

In the ordinarily used process of stabilization, the unpurified cellulose nitrate from the nitrating centrifuges is boiled with approximately 10 to 15 times its weight of water containing approximately 0.25% acid calculated as sulphuric for from 10 to- 25 hours. Where a particularly high degree of stabilization is not necessary the 10-hour boiling period is usually sufiicient.

Longer acid-boiling periods are required, however, on products which are either difficult to stabilize or in which the specifications call for a high degree of stabilization. After the acid boiling treatment, the acidified water is drained from the cellulose nitrate, fresh water added, and the boiling again carried out for a period ranging from 2 to 5 hours. This fresh water boiling from 2 to 5 hours is repeated, using fresh water each time from 2 to 4 times, depending upon the condition of the product and the desired stability.

The stability of cellulose nitrate is determined by certain laboratory tests of which there are a number such as the German test and the Bergmann-Junk test. In the German test the dry cellulose nitrate is placed in a test tube in which a piece of methyl violet paper is suspended. The test tube and its contents are then placed in a bath and heated to a temperature of l34.5 C.

plus or minus 0.5 C. Observations of color change on the paper are made at short intervals of time. The time required for the change in color is known or designated as the stability. A cellulose nitrate which will cause the methyl violet paper to change color after 10 minutes Application July 13, 1935, Serial heating is considered of low stability. One which requires 35 minutes is considered of good stability. One which requires minutes is of exceptionally high stability. The German test for stability of cellulose nitrate is described in A. S. T. M. specifications for soluble cellulose nitrate D-30131-T.

For some uses of cellulose nitrate, even very small traces of adherent acidity are harmful. The practice in connection with purification of such materials has been to subject them to long and expensive washings. Treatments with dilute solutions of sodium carbonate, etc., will remove this acidity but have the disadvantage of producing an undesirable brownish or yellow color in the finished product.

In testing the finished cellulose nitrate for acidity, a given quantity is dissolved in a mixture of ether and alcohol and the acidity titrated with standard alkali using methyl red as an indicator. For some purposes, the acidity of one gram of purified cellulose nitrate must not exceed that which can be neutralized by 0.015 cc. normal alkali solution.

This invention has as an object the provision of a process for reducing the residual acidity in cellulose nitrate. A further object is to provide a process for increasing the stability of purified cellulose nitrate by a special stabilization treatment which eliminates the long and tedious treatments now necessary. A further object is the reduction of the inherent acidity of a stabilized cellulose nitrate without the production of undesirable color in the finished product.

These objects are accomplished by the following invention through the treatment of the unpurified cellulose nitrate as obtained from the nitrating centrifuges after the removal of the adherent acids with a dilute solution of an alkali or alkaline earth salt of nitrous acid.

In carrying out the invention, the acid-wet cellulose nitrate having a Nitrogen content of 11.9% from the nitrating centrifuges is first washed with two or three changes of water, the last wash being made slightly alkaline, until the final water has an approximate pH value of 5 or higher. This is to remove the major portion of the adherent strong acids which can be removed by washing. 'I'hecellulose nitrate is then boiled with 10 to 15 times its weight of a 0.05% to 1.0% solution of an alkali or alkaline earth nitrite for from 3 to 4 hours. tially free from nitrite by means of 2 to 3 changes of fresh water. Cellulose nitrate so stabilized It is then washed substanhas a low degree of acidity and a high degree of stability.

As a commercial practice example of the invention, the following is given: 3000 lbs. of cellulose nitrate having a nitrogen content of 12.5% from the nitrating centrifuges are washed with approximately 5,000 gallons of water for about 20 minutes with tho-rough agitation. The water is drained from the cellulose nitrate and the washing repeated with 5,000 gallons of water containing approximately lbs. of sodium carbonate. This wash water is drained and the cellulose nitrate is then boiled from 3 to 4 hours with 5,000 gallons of a 0.1% solution of sodium nitrite. S0 dium carbonate is added to the second water wash merely to neutralize the adherent free acid remaining from the nitrating acid. This step is necessary in order not to decompose the sodium nitrite which is later used. After the 3 to 4 hours boiling with sodium nitrite solution the liquid is drained from the cellulose nitrate and the latter washed in 2 changes of 5,000 gallons each of fresh water for -hour periods.

While in the specific example given above sodium nitrite is used, other nitrites of the alkali or alkaline earth group can be satisfactorily used for example potassium nitrite, lithium nitrite, caesium nitrite, calcium nitrite, barium nitrite and strontium nitrite. The choice of the nitrite to be used will depend on the availability and economics involved. The amount of nitrite used can also be varied depending largely on the residual acidity of the cellulose nitrate which is to be treated. Based on the cellulose nitrate to be treated, a quantity of nitrite equal to about 1% has been found to yield satisfactory results where the pH value is above 5. For higher acidities, a greater quantity of nitrite may be required even as high as 3 to 4%. Greater amounts than this may be used but are not necessary for the accomplishment of the invention. Smaller amounts to as low as 0.5% of nitrite based on the cellulose nitrate to be treated are operative. The nitrite is always used in aqueous solutions and the cellulose nitrate treated with 10-15 times its weight of this solution.

The following has been advanced as suggesting the probable theory of the invention:

Some of the unstable bodies in cellulose nitrate are said to be compounds of cellulose or cellulose nitrate with sulphuric acid (cellulose-sulphate and cellulose-sulphate-nitrate). The sodium nitrite, being a salt of a weaker acid than sulphuric acid, may react to replace the sulphuric acid radical with the nitrous acid radical and the new compound then may be decomposed upon boiling.

This invention has been used in the production of various types of cellulose nitrate as commonly used in the arts.

In the ordinary nitrating procedure in which the nitrating acids are merely fortified for reuse, large quantities of impurities accumulate in the nitrating mixture. Some of these impurities, notably the nitrosyl-sulphuric acid, cause the finished cellulose nitrate to have a high degree of inherent acidity which is difficult of removal.

For example, an ordinarily nitrated cellulose nitrate stabilized by boiling for 10 hours with acidulated water, and given two 2-hour additional boils each with fresh Water, as described above will give a finished cellulose nitrate having a stability of 35 minutes by the above mentioned German test and an inherent acidity such that 1 gram will require about 0.02 cc. of normal alkali for neutralization. The same cellulose nitrate treated as in the above example of the invention will likewise have a stability of 35 minutes in the German test, and will have an acidity such that 1 gram will require only 0.015 cc. of normal alkali for neutralization.

Further advantages of this invention may be summarized as follows: The cycle of operation is shortened so as to increase the capacity of stabilizing equipment by threefold. The steam consumption for heating is only about onethird of that required in the present practice because of the use of a smaller quantity of wash water. The economic advantages in the process are, therefore, quite evident.

Since the presence of high nitrosyl-sulphuric acid in nitrating acids which are merely fortified for reuse, and not frequently regenerated, pro duces a cellulose nitrate which is high in inherent acidity, and since such high acidity is difficult or impossible to remove by the older method, and is satisfactorily removed by this invention, the process here described avoids the frequent regeneration of nitrating acids in cases where the finished cellulose nitrate must have an unusually low acidity.

It is apparent that many widely different embodiments of this invention may be made without departing from the spirit and scope thereof; and, therefore, it is not intended to be limited except as indicated in the appended claims.

I claim:

1. Process for stabilizing cellulose nitrate which comprises removing the excess acid from the unpurified nitrated cellulose by repeated washings and thereafter treating the cellulose nitrate with an aqueous solution of a nitrite of a metal of the alkali or alkaline earth group having an alkaline reaction.

2. In a process for stabilizing cellulose nitrate containing adherent and inherent acidity, the improvement which comprises removing the adherent acidity by treating the same with a solution of sodium carbonate and decreasing the inherent acidity by treating with an aqueous solution of a nitrite of a metal of the alkali or alkaline earth group.

3. A process for making a cellulose nitrate of high stability which comprises removing the excess acid from the nitrated cellulose and thereafter treating the same with a 0.05 to 1.0% solution of sodium nitrite and then removing sodium nitrite by washing with water.

4. Process of producing cellulose nitrate of high stability which comprises nitrating cellulose, centrifuging off the excess nitrating acids, washing with a large volume of water, repeating the washing step with water containing sodium carbonate to neutralize the excess acid, and then boiling for from 3 to 4 hours with a large volume of .1% solution of sodium nitrite and thereafter removing the said solution.

LOUIS S. BAKER. 

